1. Field of the Invention
This invention relates to a process for preparing an article bearing an improved anti-fog layer-by-layer coating and to the articles bearing an anti-fog layer-by layer coating obtained by said process, more particularly optical and ophthalmic articles, especially ophthalmic lenses for eyeglasses.
The invention is based on the use of a specific intermediate coating enhancing anti-fog properties and adhesion of said anti-fog layer-by-layer coating, along with a specific heating step of the layer-by-layer coating.
2. Description of Related Art
It is known in the art that a layer-by-layer (LbL) coating can be assembled on a substrate from species having opposite charges. More precisely, positively and negatively charged polyelectrolytes can be alternately deposited on a substrate.
For this purpose, at least two different polyelectrolyte solutions having opposite charges, or a polyelectrolyte solution and a nanoparticle solution having opposite charges, may be used to form the LbL coating.
As known in the art, a polyelectrolyte is a polymer having a substantial portion of its repeating units bearing an electrolyte group. These groups are ionic or ionizable groups, especially in aqueous solutions.
Other known LbL coatings include a plurality of bilayers alternately comprising oppositely charged nanoparticles.
By selecting the materials of the layers and the deposition process conditions, such films can be anti-reflective, hydrophilic or superhydrophilic, hydrophobic or superhydrophobic.
LbL coatings having hydrophilic properties may also have anti-fog properties.
US 2007/0104922 describes superhydrophilic LbL coatings that can be antireflective and anti-fog. i.a. poly(allylamine hydrochloride)/SiO2 LbL coatings.
A disadvantage of the anti-fog LbL coatings is that such coatings exhibit generally poor mechanical properties, especially poor adhesion to organic substrates, either naked or coated by classical hard coats. Mechanical properties of LbL coatings have been increased by calcination treatment, generally at high temperature (typically 550° C.).
A disadvantage associated to this technique is that it cannot be applied on any organic substrates and is only adapted to substrates that can withstand high temperature like glass substrates.
In the article “Hydrothermal Treatment of Nanoparticle Thin Films for Enhanced Mechanical Durability” Z. Gemici et al., Langmuir 2008, 24, 2168-2177, a hydrothermal treatment implemented at around 125° C., of different LbL coatings is described in order to improve mechanical durability of these coatings and avoid delamination, especially on a polycarbonate (PC) substrate.
A typical example of a LbL coating is either a polymer-nanoparticle coating made alternatively from a positively charged poly(diallyldimethyl ammonium chloride) and negatively charged silica nanoparticles, or an all-nanoparticle coating assembled alternatively from positively charged 3-aminopropyl silane modified silica (or titania) nanoparticles and negatively charged silica nanoparticles. After a hydrothermal treatment, such LbL coating has improved abrasion resistance.
US 2008/0038458 describes a hydrothermal calcination of TiO2/SiO2 LbL coatings, typically at a pressure in the range of 10 psi to 30 psi at temperature less than 500° C.
One disadvantage of the technique, along with the necessity of using an autoclave, is that the hydrothermal treatment affects the anti-fog properties of the coating, as explained in US 2008/038458 paragraph [0046]: the coating can lose its anti-fog properties.
Consequently, it is desirable to provide a new and simple process of preparation of anti-fog LbL coatings having good or improved anti-fog properties, along with good mechanical properties such as improved adhesion to the substrate and/or improved abrasion resistance.
U.S. Pat. No. 6,984,262 describes a coating composition adapted to enhance the adhesion of a polymeric coating or film applied to a substrate.
This adhesive coating comprises at least one specific silane coupling agent, at least partial hydrolyzates thereof in a concentration greater than 25% and an adhesion enhancing amount of an epoxy-containing material comprising at least two epoxy groups.
There is no disclosure of an anti-fog LbL coating as the polymeric coating and no disclosure that the adhesive coating can improve the anti-fog properties of a LbL system.